1================================================================
2Documentation for Kdump - The kexec-based Crash Dumping Solution
3================================================================
45This document includes overview, setup and installation, and analysis
6information.
78Overview
9========
1011Kdump uses kexec to quickly boot to a dump-capture kernel whenever a
12dump of the system kernel's memory needs to be taken (for example, when
13the system panics). The system kernel's memory image is preserved across
14the reboot and is accessible to the dump-capture kernel.
1516You can use common commands, such as cp and scp, to copy the
17memory image to a dump file on the local disk, or across the network to
18a remote system.
1920Kdump and kexec are currently supported on the x86, x86_64, ppc64, ia64,
21s390x and arm architectures.
2223When the system kernel boots, it reserves a small section of memory for
24the dump-capture kernel. This ensures that ongoing Direct Memory Access
25(DMA) from the system kernel does not corrupt the dump-capture kernel.
26The kexec -p command loads the dump-capture kernel into this reserved
27memory.
2829On x86 machines, the first 640 KB of physical memory is needed to boot,
30regardless of where the kernel loads. Therefore, kexec backs up this
31region just before rebooting into the dump-capture kernel.
3233Similarly on PPC64 machines first 32KB of physical memory is needed for
34booting regardless of where the kernel is loaded and to support 64K page
35size kexec backs up the first 64KB memory.
3637For s390x, when kdump is triggered, the crashkernel region is exchanged
38with the region [0, crashkernel region size] and then the kdump kernel
39runs in [0, crashkernel region size]. Therefore no relocatable kernel is
40needed for s390x.
4142All of the necessary information about the system kernel's core image is
43encoded in the ELF format, and stored in a reserved area of memory
44before a crash. The physical address of the start of the ELF header is
45passed to the dump-capture kernel through the elfcorehdr= boot
46parameter. Optionally the size of the ELF header can also be passed
47when using the elfcorehdr=[size[KMG]@]offset[KMG] syntax.
484950With the dump-capture kernel, you can access the memory image through
51/proc/vmcore. This exports the dump as an ELF-format file that you can
52write out using file copy commands such as cp or scp. Further, you can
53use analysis tools such as the GNU Debugger (GDB) and the Crash tool to
54debug the dump file. This method ensures that the dump pages are correctly
55ordered.
565758Setup and Installation
59======================
6061Install kexec-tools
62-------------------
63641) Login as the root user.
65662) Download the kexec-tools user-space package from the following URL:
6768http://kernel.org/pub/linux/utils/kernel/kexec/kexec-tools.tar.gz6970This is a symlink to the latest version.
7172The latest kexec-tools git tree is available at:
7374git://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
75and
76http://www.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git7778There is also a gitweb interface available at
79http://www.kernel.org/git/?p=utils/kernel/kexec/kexec-tools.git8081More information about kexec-tools can be found at
82http://horms.net/projects/kexec/83843) Unpack the tarball with the tar command, as follows:
8586 tar xvpzf kexec-tools.tar.gz
87884) Change to the kexec-tools directory, as follows:
8990 cd kexec-tools-VERSION
91925) Configure the package, as follows:
9394 ./configure
95966) Compile the package, as follows:
9798 make
991007) Install the package, as follows:
101102 make install
103104105Build the system and dump-capture kernels
106-----------------------------------------
107There are two possible methods of using Kdump.
1081091) Build a separate custom dump-capture kernel for capturing the
110 kernel core dump.
1111122) Or use the system kernel binary itself as dump-capture kernel and there is
113 no need to build a separate dump-capture kernel. This is possible
114 only with the architectures which support a relocatable kernel. As
115 of today, i386, x86_64, ppc64, ia64 and arm architectures support relocatable
116 kernel.
117118Building a relocatable kernel is advantageous from the point of view that
119one does not have to build a second kernel for capturing the dump. But
120at the same time one might want to build a custom dump capture kernel
121suitable to his needs.
122123Following are the configuration setting required for system and
124dump-capture kernels for enabling kdump support.
125126System kernel config options
127----------------------------
1281291) Enable "kexec system call" in "Processor type and features."
130131 CONFIG_KEXEC=y
1321332) Enable "sysfs file system support" in "Filesystem" -> "Pseudo
134 filesystems." This is usually enabled by default.
135136 CONFIG_SYSFS=y
137138 Note that "sysfs file system support" might not appear in the "Pseudo
139 filesystems" menu if "Configure standard kernel features (for small
140 systems)" is not enabled in "General Setup." In this case, check the
141 .config file itself to ensure that sysfs is turned on, as follows:
142143 grep 'CONFIG_SYSFS' .config
1441453) Enable "Compile the kernel with debug info" in "Kernel hacking."
146147 CONFIG_DEBUG_INFO=Y
148149 This causes the kernel to be built with debug symbols. The dump
150 analysis tools require a vmlinux with debug symbols in order to read
151 and analyze a dump file.
152153Dump-capture kernel config options (Arch Independent)
154-----------------------------------------------------
1551561) Enable "kernel crash dumps" support under "Processor type and
157 features":
158159 CONFIG_CRASH_DUMP=y
1601612) Enable "/proc/vmcore support" under "Filesystems" -> "Pseudo filesystems".
162163 CONFIG_PROC_VMCORE=y
164 (CONFIG_PROC_VMCORE is set by default when CONFIG_CRASH_DUMP is selected.)
165166Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
167--------------------------------------------------------------------
1681691) On i386, enable high memory support under "Processor type and
170 features":
171172 CONFIG_HIGHMEM64G=y
173 or
174 CONFIG_HIGHMEM4G
1751762) On i386 and x86_64, disable symmetric multi-processing support
177 under "Processor type and features":
178179 CONFIG_SMP=n
180181 (If CONFIG_SMP=y, then specify maxcpus=1 on the kernel command line
182 when loading the dump-capture kernel, see section "Load the Dump-capture
183 Kernel".)
1841853) If one wants to build and use a relocatable kernel,
186 Enable "Build a relocatable kernel" support under "Processor type and
187 features"
188189 CONFIG_RELOCATABLE=y
1901914) Use a suitable value for "Physical address where the kernel is
192 loaded" (under "Processor type and features"). This only appears when
193 "kernel crash dumps" is enabled. A suitable value depends upon
194 whether kernel is relocatable or not.
195196 If you are using a relocatable kernel use CONFIG_PHYSICAL_START=0x100000
197 This will compile the kernel for physical address 1MB, but given the fact
198 kernel is relocatable, it can be run from any physical address hence
199 kexec boot loader will load it in memory region reserved for dump-capture
200 kernel.
201202 Otherwise it should be the start of memory region reserved for
203 second kernel using boot parameter "crashkernel=Y@X". Here X is
204 start of memory region reserved for dump-capture kernel.
205 Generally X is 16MB (0x1000000). So you can set
206 CONFIG_PHYSICAL_START=0x1000000
2072085) Make and install the kernel and its modules. DO NOT add this kernel
209 to the boot loader configuration files.
210211Dump-capture kernel config options (Arch Dependent, ppc64)
212----------------------------------------------------------
2132141) Enable "Build a kdump crash kernel" support under "Kernel" options:
215216 CONFIG_CRASH_DUMP=y
2172182) Enable "Build a relocatable kernel" support
219220 CONFIG_RELOCATABLE=y
221222 Make and install the kernel and its modules.
223224Dump-capture kernel config options (Arch Dependent, ia64)
225----------------------------------------------------------
226227- No specific options are required to create a dump-capture kernel
228 for ia64, other than those specified in the arch independent section
229 above. This means that it is possible to use the system kernel
230 as a dump-capture kernel if desired.
231232 The crashkernel region can be automatically placed by the system
233 kernel at run time. This is done by specifying the base address as 0,
234 or omitting it all together.
235236 crashkernel=256M@0
237 or
238 crashkernel=256M
239240 If the start address is specified, note that the start address of the
241 kernel will be aligned to 64Mb, so if the start address is not then
242 any space below the alignment point will be wasted.
243244Dump-capture kernel config options (Arch Dependent, arm)
245----------------------------------------------------------
246247- To use a relocatable kernel,
248 Enable "AUTO_ZRELADDR" support under "Boot" options:
249250 AUTO_ZRELADDR=y
251252Extended crashkernel syntax
253===========================
254255While the "crashkernel=size[@offset]" syntax is sufficient for most
256configurations, sometimes it's handy to have the reserved memory dependent
257on the value of System RAM -- that's mostly for distributors that pre-setup
258the kernel command line to avoid a unbootable system after some memory has
259been removed from the machine.
260261The syntax is:
262263 crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
264 range=start-[end]
265266Please note, on arm, the offset is required.
267 crashkernel=<range1>:<size1>[,<range2>:<size2>,...]@offset
268 range=start-[end]
269270 'start' is inclusive and 'end' is exclusive.
271272For example:
273274 crashkernel=512M-2G:64M,2G-:128M
275276This would mean:
277278 1) if the RAM is smaller than 512M, then don't reserve anything
279 (this is the "rescue" case)
280 2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
281 3) if the RAM size is larger than 2G, then reserve 128M
282283284285Boot into System Kernel
286=======================
2872881) Update the boot loader (such as grub, yaboot, or lilo) configuration
289 files as necessary.
2902912) Boot the system kernel with the boot parameter "crashkernel=Y@X",
292 where Y specifies how much memory to reserve for the dump-capture kernel
293 and X specifies the beginning of this reserved memory. For example,
294 "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
295 starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
296297 On x86 and x86_64, use "crashkernel=64M@16M".
298299 On ppc64, use "crashkernel=128M@32M".
300301 On ia64, 256M@256M is a generous value that typically works.
302 The region may be automatically placed on ia64, see the
303 dump-capture kernel config option notes above.
304 If use sparse memory, the size should be rounded to GRANULE boundaries.
305306 On s390x, typically use "crashkernel=xxM". The value of xx is dependent
307 on the memory consumption of the kdump system. In general this is not
308 dependent on the memory size of the production system.
309310 On arm, use "crashkernel=Y@X". Note that the start address of the kernel
311 will be aligned to 128MiB (0x08000000), so if the start address is not then
312 any space below the alignment point may be overwritten by the dump-capture kernel,
313 which means it is possible that the vmcore is not that precise as expected.
314315316Load the Dump-capture Kernel
317============================
318319After booting to the system kernel, dump-capture kernel needs to be
320loaded.
321322Based on the architecture and type of image (relocatable or not), one
323can choose to load the uncompressed vmlinux or compressed bzImage/vmlinuz
324of dump-capture kernel. Following is the summary.
325326For i386 and x86_64:
327 - Use vmlinux if kernel is not relocatable.
328 - Use bzImage/vmlinuz if kernel is relocatable.
329For ppc64:
330 - Use vmlinux
331For ia64:
332 - Use vmlinux or vmlinuz.gz
333For s390x:
334 - Use image or bzImage
335For arm:
336 - Use zImage
337338If you are using a uncompressed vmlinux image then use following command
339to load dump-capture kernel.
340341 kexec -p <dump-capture-kernel-vmlinux-image> \
342 --initrd=<initrd-for-dump-capture-kernel> --args-linux \
343 --append="root=<root-dev> <arch-specific-options>"
344345If you are using a compressed bzImage/vmlinuz, then use following command
346to load dump-capture kernel.
347348 kexec -p <dump-capture-kernel-bzImage> \
349 --initrd=<initrd-for-dump-capture-kernel> \
350 --append="root=<root-dev> <arch-specific-options>"
351352If you are using a compressed zImage, then use following command
353to load dump-capture kernel.
354355 kexec --type zImage -p <dump-capture-kernel-bzImage> \
356 --initrd=<initrd-for-dump-capture-kernel> \
357 --dtb=<dtb-for-dump-capture-kernel> \
358 --append="root=<root-dev> <arch-specific-options>"
359360361Please note, that --args-linux does not need to be specified for ia64.
362It is planned to make this a no-op on that architecture, but for now
363it should be omitted
364365Following are the arch specific command line options to be used while
366loading dump-capture kernel.
367368For i386, x86_64 and ia64:
369 "1 irqpoll maxcpus=1 reset_devices"
370371For ppc64:
372 "1 maxcpus=1 noirqdistrib reset_devices"
373374For s390x:
375 "1 maxcpus=1 cgroup_disable=memory"
376377For arm:
378 "1 maxcpus=1 reset_devices"
379380Notes on loading the dump-capture kernel:
381382* By default, the ELF headers are stored in ELF64 format to support
383 systems with more than 4GB memory. On i386, kexec automatically checks if
384 the physical RAM size exceeds the 4 GB limit and if not, uses ELF32.
385 So, on non-PAE systems, ELF32 is always used.
386387 The --elf32-core-headers option can be used to force the generation of ELF32
388 headers. This is necessary because GDB currently cannot open vmcore files
389 with ELF64 headers on 32-bit systems.
390391* The "irqpoll" boot parameter reduces driver initialization failures
392 due to shared interrupts in the dump-capture kernel.
393394* You must specify <root-dev> in the format corresponding to the root
395 device name in the output of mount command.
396397* Boot parameter "1" boots the dump-capture kernel into single-user
398 mode without networking. If you want networking, use "3".
399400* We generally don' have to bring up a SMP kernel just to capture the
401 dump. Hence generally it is useful either to build a UP dump-capture
402 kernel or specify maxcpus=1 option while loading dump-capture kernel.
403404* For s390x there are two kdump modes: If a ELF header is specified with
405 the elfcorehdr= kernel parameter, it is used by the kdump kernel as it
406 is done on all other architectures. If no elfcorehdr= kernel parameter is
407 specified, the s390x kdump kernel dynamically creates the header. The
408 second mode has the advantage that for CPU and memory hotplug, kdump has
409 not to be reloaded with kexec_load().
410411* For s390x systems with many attached devices the "cio_ignore" kernel
412 parameter should be used for the kdump kernel in order to prevent allocation
413 of kernel memory for devices that are not relevant for kdump. The same
414 applies to systems that use SCSI/FCP devices. In that case the
415 "allow_lun_scan" zfcp module parameter should be set to zero before
416 setting FCP devices online.
417418Kernel Panic
419============
420421After successfully loading the dump-capture kernel as previously
422described, the system will reboot into the dump-capture kernel if a
423system crash is triggered. Trigger points are located in panic(),
424die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).
425426The following conditions will execute a crash trigger point:
427428If a hard lockup is detected and "NMI watchdog" is configured, the system
429will boot into the dump-capture kernel ( die_nmi() ).
430431If die() is called, and it happens to be a thread with pid 0 or 1, or die()
432is called inside interrupt context or die() is called and panic_on_oops is set,
433the system will boot into the dump-capture kernel.
434435On powerpc systems when a soft-reset is generated, die() is called by all cpus
436and the system will boot into the dump-capture kernel.
437438For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
439"echo c > /proc/sysrq-trigger" or write a module to force the panic.
440441Write Out the Dump File
442=======================
443444After the dump-capture kernel is booted, write out the dump file with
445the following command:
446447 cp /proc/vmcore <dump-file>
448449450Analysis
451========
452453Before analyzing the dump image, you should reboot into a stable kernel.
454455You can do limited analysis using GDB on the dump file copied out of
456/proc/vmcore. Use the debug vmlinux built with -g and run the following
457command:
458459 gdb vmlinux <dump-file>
460461Stack trace for the task on processor 0, register display, and memory
462display work fine.
463464Note: GDB cannot analyze core files generated in ELF64 format for x86.
465On systems with a maximum of 4GB of memory, you can generate
466ELF32-format headers using the --elf32-core-headers kernel option on the
467dump kernel.
468469You can also use the Crash utility to analyze dump files in Kdump
470format. Crash is available on Dave Anderson's site at the following URL:
471472http://people.redhat.com/~anderson/473474Trigger Kdump on WARN()
475=======================
476477The kernel parameter, panic_on_warn, calls panic() in all WARN() paths. This
478will cause a kdump to occur at the panic() call. In cases where a user wants
479to specify this during runtime, /proc/sys/kernel/panic_on_warn can be set to 1
480to achieve the same behaviour.
481482Contact
483=======
484485Vivek Goyal (vgoyal@redhat.com)
486Maneesh Soni (maneesh@in.ibm.com)